1. Technical Field
This invention relates to a composite substrate for use in light emitting display devices and planar light sources and a method for preparing the same, and especially to an EL panel of AC drive type EL device using a high-permittivity ceramic layer as the insulating layer.
2. Background Art
EL devices are on commercial use as backlight in liquid crystal displays (LCD) and watches.
The EL devices utilize the phenomenon that a material emits light upon application of an electric field, known as electroluminescent phenomenon.
The EL devices include dispersion type EL devices of the structure that a dispersion of powder luminescent material in organic material or enamel is sandwiched between electrode layers, and thin-film type EL devices in which a light emitting thin film sandwiched between two electrode layers and two insulating thin films is formed on an electrically insulating substrate. For each type, the drive modes include DC voltage drive mode and AC voltage drive mode. The dispersion type EL devices are known from the past and have the advantage of easy manufacture, but their use is limited because of a low luminance and a short lifetime. On the other hand, the thin-film type EL devices are currently on widespread use on account of a high luminance and a long lifetime.
FIG. 19 shows the structure of a dual insulated thin-film EL device as a typical prior art thin-film type EL device. This thin-film EL device includes a transparent substrate 21 of a sheet glass customarily used in liquid crystal displays or plasma display panels (PDP), a transparent electrode layer 22 formed of ITO or the like in a predetermined stripe pattern to a thickness of about 0.2 to 1 μm, a thin-film transparent first insulator layer 23, a light emitting layer 24 having a thickness of about 0.2 to 1 μm, and a thin-film second insulator layer 25, and a metal electrode layer 26 of Al thin film or the like which is patterned into stripes extending perpendicular to the transparent electrode layer 22. A voltage from a power supply 30 is selectively applied to a specific light-emitting material selected in the matrix formed by the transparent electrode layer 22 and the metal electrode layer 26, whereby the light-emitting material in the selected pixel emits light which comes out from the substrate 21 side. The thin-film insulator layers 23, 25 have a function of restricting the current flow through the light emitting layer 24 in order to restrain breakdown of the thin-film EL device and act so as to provide stable light-emitting characteristics. Thus thin-film EL devices of this structure are on widespread commercial use.
Nevertheless, for these thin-film EL devices, a structural problem remains still unsolved. Specifically, since the insulator layer is formed of a thin film, it is difficult to manufacture displays having large surface areas while completely eliminating steps at the edge of a transparent electrode pattern and avoiding defects in the thin-film insulator introduced by debris or the like in the manufacturing process. This leaves a problem that the light emitting layer fails on account of a local drop of dielectric strength. Such defectives impose a fatal problem to display devices and create a substantial barrier against the widespread commercial application of thin-film EL devices as large-area displays, in contrast to liquid crystal displays and plasma displays.
To solve the problem of defects in the thin-film insulator, JP-B 7-44072 discloses an EL device which uses an electrically insulating ceramic substrate as the substrate and a thick-film dielectric material instead of the thin-film insulator underlying the light emitting layer. Since the EL device of the above patent is constructed such that light emitted by the light emitting layer is extracted from the upper side remote from the substrate as opposed to prior art thin-film EL devices, a transparent electrode layer is formed on the upper side.
Further, in this EL device, the thick-film dielectric layer is formed to a thickness of several tens to several hundreds of microns, which is several hundred to several thousand times of the thickness of the thin-film insulator layer. This minimizes the potential of breakdown during initial operation which is otherwise caused by steps of electrodes and pinholes formed by debris in the manufacturing process. Meanwhile, the use of such a thick-film dielectric layer entails a problem that the effective voltage applied across the light emitting layer drops. For example, the above-referred JP-B 7-44072 overcomes this problem by using a complex perovskite high-permittivity material in the dielectric layer.
However, the light emitting layer formed on the thick-film dielectric layer has a thickness of several hundreds of nanometers which is merely about {fraction (1/100)} of that of the thick-film dielectric layer. This requires that the thick-film dielectric layer on the surface be smooth at a level below the thickness of the light emitting layer although a conventional thick-film procedure is difficult to form a dielectric layer having a fully smooth surface.
Specifically, the thick-film dielectric layer is essentially constructed of a ceramic material obtained using a powder raw material. Then intense sintering generally brings a volume contraction of about 30 to 40%. Unfortunately, although customary ceramics consolidate through three-dimensional volume contraction upon sintering, thick-film ceramics formed on substrates cannot contract in the in-plane directions of the substrate under restraint by the substrate, and is allowed for only one-dimensional volume contraction in the thickness direction. For this reason, sintering of the thick-film dielectric layer proceeds insufficiently, resulting in an essentially porous body. Moreover, since the surface roughness of the thick film is not reduced below the crystal grain size of the polycrystalline sintered body, its surface has asperities greater than the submicron size.
On such an uneven surface of the dielectric layer, a light emitting layer cannot be uniformly formed by vapor phase deposition techniques such as evaporation or sputtering. It is then impossible to effectively apply an electric field across an uneven light emitting layer, resulting in a reduction of effective luminous area. On account of local unevenness of film thickness, the light emitting layer undergoes partial breakdown, resulting in a lowering of emission luminance. Moreover, since the film thickness has large local variations, the strength of the electric field applied across the light emitting layer has large local variations as well, failing to provide a definite emission voltage threshold.
To solve these and other problems, for example, JP-A 7-50197 discloses a procedure of improving surface smoothness by stacking on a thick-film dielectric of lead niobate a high-permittivity layer of lead titanate zirconate or the like to be formed by the sol-gel technique. As shown in FIG. 20, an electrode 12 is formed on a substrate 11, a thick-film dielectric layer 13 is formed thereon, and a smoothing layer 14 of lead titanate zirconate or the like is then formed by the sol-gel technique, thereby improving surface smoothness.
However, when a smoothing layer is formed on the surface of a thick-film dielectric layer having a thickness of about several tens to several hundreds of microns which is porous, microcracks generate in the smoothing layer. The cracked areas become less insulating, making it difficult to ensure stable operation over a long term.
Moreover, the thick-film dielectric layer can locally include asperities of a size larger than the above-described surface roughness. Once such large asperities are formed, it becomes difficult to form a completely smooth layer thereon by the sol-gel technique.
When a thick film having local asperities is leveled out by forming PZT thereon by the sol-gel technique and firing, the surface roughness following the smoothening has a substantial variation. The variation of surface roughness is eventually reflected by a luminous variation at low luminance during EL light emission and in serious cases, can cause the smoothing layer to generate cracks. Cracks become the cause of abnormal light emission known as bright spots. In either case, the variation of surface roughness causes a variation of light emission.